The AI merge path treated a missing task branch as a benign no-op. That's
correct when the task was never executed or already merged (branch cleaned up
on a re-process), but if the task WAS executed (a baseCommitSha was recorded)
and has no recorded merge, the branch should still exist — its work appears
lost. Now that case throws instead of silently marking the task done; the
benign cases still finalize as a no-op (reason "already-merged" / "no-branch").
Tests: executed+never-merged → throws; already-merged → no-op done;
never-executed → no-op done.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
be0be507b3